In children tendon tears are common at the site of insertion to the bone, these are known as avulsion fractures. These occur due to childrens bones are not a developed as tendons thus acting as a weak point until complete ossification. In terms of adults it really does depend on the extrensic factors eg.direction and degree of force, but an assumption that can be made is that at the insertion and origin points the tendenous sheet is at its thinnest so one would expect rupture at these sites first.

avulsion fractures are not tendon tears!Most tears occur at the musculo-tendonous junction as it is there where the adhesion surface is the greatest and the amount of tendon fiber onto other tissue fiber (the collagen interface with muscle) is the smallest. These tears are often small individually but multiple throughout the area and do not represent a loss of continuity, i.e. the tendon has not separated from the muscle. It more is like a series of rents in the fabric if you will